Thermistors have long been used to measure mass air flow. If power is supplied to a thermistor, its temperature increases, and its resistance changes. Moving air carries heat away from the thermistor, resulting in a variable amount of power to maintain a predetermined temperature of the thermistor.
If ambient temperature and pressure are known, the thermistor can be calibrated to reflect air velocity at Standard Temperature and Pressure conditions. Heat transfer depends not only on the mass air flow, but also on the difference between the temperature of the heated thermistor and the ambient air temperature. A separate, unheated thermistor may commonly be used to measure the ambient air temperature.
Commercial thermistors are heated, and mass air flow is computed based on the amount of energy required to keep the thermistor at a known state. Unheated sensors are required to measure ambient air temperature, increasing the complexity of mass air flow sensors. Calibration over a large range of temperatures may also be required where the ambient temperature may change significantly. Power consumption may also be high, since the thermistor is continuously heated.